In order to catalytically accelerate chemical reactions in fluid streams, one uses catalysts that are applied on or contained in catalyst bases. Usually, such bases comprise at least one contact area of a porous material. The surface that is made of and enlarged by the porous material includes active centers favoring a reaction of the reactants carried by the fluid stream. The contact area is understood as a macroscopic boundary of the base material adjacent to the fluid stream. That contact area is to be distinguished from the pore wall areas that have smaller, sometimes microscopic dimensions and are formed by areas defining the boundaries of the pore spaces/volumes. The pore wall areas of such pores leading into an opening in the contact area form one surface with the contact area and enlarge the surface that is accessible by the fluid stream.
The streaming fluid and the reactants (within the fluid) enter (diffuse) into the pore system that is formed by the pores; they come into contact with the active centers located at the pore wall areas and there they are adsorbed. In this process, the reactants react and the reaction products are released by desorption and diffuse out of the pore system back into the fluid stream. Such catalyst bases are in particular used in processes, if exhaust gases are to be cleaned from undesired substances carried within the gases, e.g. when cleaning the flue gas of combustion power plants.
Burning fossil fuels (or waste and/or biomass) produces air pollutants. It is desirable to remove the air pollutants because of their negative effect on the environment. Apart from the dusts that can be removed from flue gas streams by dust removing devices, sulfur compounds are removed by desulfurization plants. However, fossil fuels contain in particular nitrogen compounds that are transformed to nitrogen oxides in the flue gas. Furthermore, a portion of the nitrogen of the combustion air is changed to nitrogen oxide under combustion conditions. Accordingly, in respect of the production of nitrogen oxides, a distinction is made between thermal formation, prompt (direct) nitrogen oxide formation and nitrogen oxide formation from fuel nitrogen. For environmental reasons, the portion of nitrogen oxides (NOx) in exhaust gases should be reduced.
Apart from reducing the formation of nitrogen oxide by primary measures, i.e. measures affecting the fueling, it is becoming more and more common for power plants to also use secondary measures, i.e. removing NOx compounds from the flue gases.
Apart from other methods (e.g. selective non-catalytic reduction), catalytic reduction methods are in particular relevant due to their large-scale applicability.
The so-called selective catalytic reduction methods (SCR) are used for the NOx reduction on a large industrial scale. When those methods are used, NOx is transformed to water and nitrogen using NH3 (ammonia). In the presence of catalysts, the reaction runs faster and/or at a lower temperature and therefore it is also suitable for high flue gas stream velocities. The used catalysts usually consist of catalyst bases (substrates/carriers) of ceramic base materials, in which active metal compounds have been homogeneously introduced or on whose surfaces active metal compounds that combine with the base material have been applied (usually a simple coating cannot be used because of the mechanical stress in the flue gas stream). Frequently, the main component for the catalyst base is titanium dioxide; active centers are formed by adding vanadium, tungsten, molybdenum, copper and/or iron compounds. Zeolites can also be used as catalyst bases.
It is problematic when catalysts are used to reduce nitrogen oxides, as there are competing reactions, which are also favored by the active centers and which can in particular result in the undesired oxidization of sulfur dioxide to sulfur trioxide.
Furthermore, the activity of the catalyst (the desired nitrogen oxide reduction) as well as (due to a decrease in competition) the proportion between the desired catalytic effect and the undesired catalytic effect decreases the longer the catalyst is used. This is the reason that catalyst bases must be reactivated after a certain period of time, i.e. the catalyst base is cleaned, reaction products are removed and the base material is covered/doped with new active centers. For this purpose, a catalytically relevant substance is introduced into pores of the catalyst base using a transport fluid. The introduction is carried out through openings in the contact area, into which the pores lead (pores that do not have any fluid contact to openings in the contact area are irrelevant, since flue gas cannot enter such pores during operation). After removal of the transport fluid, the catalytically relevant substance remains on the pore wall areas of the pores and there it forms active centers.
Usually, catalysts that have been reactivated in such a way have a similar activity to newly manufactured catalysts. However, it can happen that the proportion between activity (catalysis of the nitrogen oxide reduction) and undesired reactions changes for the worse.
This is the starting point of the invention.
The purpose of the invention is to provide a method for treating a catalyst base, wherein the method can be used for the first manufacturing of catalysts as well as for their reactivation, and wherein the method makes a high activity of the catalyst possible without increasing the undesired side reactions.